Swing Arm Length

[Sputniks]

It has been mentioned the length of the Streetfighter's swingarm is considerably longer than the one installed on 1098 superbike. At the same time the description of the BST magnesium swingarm specifies that BST swingarm is longer than a stock 1098's one (while still being shorter than Streetfighter swingarm). It is being said that the magnesium swingarm made to the exact specs used in WSB bikes. Should it be assumed the longer the swinarm the better? Would it mean Streetfighter's longest swinarm of all is superior to the stock 1098 and the magnesium BST? If there is some performance gain in a longer swingarm why then Ducati didn't design 1098 with the length of the swingarm equal to the ones used in WSB or made by BST? What characteristics of the bike are effected by the length of the swinarm? How different Streetfighter would feel if its swingarm length would be the same as 1098 one?

[Barone]

I think "better or worse" depends on the expected use of the bike, a longer swingarm helps to release hp to ground but makes the bike less manageable in changing direction because it makes bike more long, contrariwise a shorter swingarm makes the bike more reactive but "shaky" at high speeds and the bike can goes one wheel easier on WOT

[pcoakle1]

The different length swingarm in the SF is probably also relating to the geometry of the bike. Front-end is set up a bit different on the SF I think.

[Sputniks]

That's clear. Thanks for the explanation. Now... Let's think what would happen (in a theory) if a shorter swingarm is installed on Streetfighter. As I know Streetfighter's front fork is angled more (forward) than on 1098's fork. With a front fork's angle increased (comparing to a superbike) and the longer swingarm the distance between the wheels (or better to say: the distance between the front wheel's and rear wheel's contact patch or point) is longer than on superbike. By other words Streetfighter's wheelbase is longer than on 1098. With a shorter swingarm and the stock front fork angle a Streetfighter's wheelbase will be shortened from the rear forward. While the center of mass would remain practically the same. A "misplaced" center of mass would probably result to too easy wheelies and yet with no increase of cornering maneuverability... If not worsened one.


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[Jonny DeWog]

It's all about wheelbase:

A short swingarm, when compressed, loses more wheelbase length due to it's arc of travel (ie as the rear wheel moves upwards - like in accelerating or hitting a bump - it moves closer to the front wheel and decreases the wheelbase).

A longer swingarm still reduces wheelbase as it moves up, but less so than a short one. The ideal swingarm is one that is "infinitely" long with a "virtual" infintie arc of travel....ie - not allowing the rear wheel to decrease the wheelbase when it moves up.

When a bike is leaned over it's radius of turn around a corner is dictated by:

1) curve of the tyres (profile)
2) wheelbase (a shorter wheelbase means smaller arc)
3) degree of lean coupled with the above two factors.

Clearly anything that upsets (changes) the wheelbase dramatically, like a short swingarm, will make cornering more difficult and less stable. hence a longer swingarm is preferable.

The SF swingarm is 35mm longer then stock 1098, but i understand the new Panigale swingarm is longer too.

jdw

[direktor]

Clearly, the longer the swing arm, the faster and better the bike handles:

[FlyingQ]

Originally Posted by Jonny DeWog

It's all about wheelbase:

A short swingarm, when compressed, loses more wheelbase length due to it's arc of travel (ie as the rear wheel moves upwards - like in accelerating or hitting a bump - it moves closer to the front wheel and decreases the wheelbase).

A longer swingarm still reduces wheelbase as it moves up, but less so than a short one. The ideal swingarm is one that is "infinitely" long with a "virtual" infintie arc of travel....ie - not allowing the rear wheel to decrease the wheelbase when it moves up.

When a bike is leaned over it's radius of turn around a corner is dictated by:

1) curve of the tyres (profile)
2) wheelbase (a shorter wheelbase means smaller arc)
3) degree of lean coupled with the above two factors.

Clearly anything that upsets (changes) the wheelbase dramatically, like a short swingarm, will make cornering more difficult and less stable. hence a longer swingarm is preferable.

The SF swingarm is 35mm longer then stock 1098, but i understand the new Panigale swingarm is longer too.

jdw

Good stuff. Thanks.

[Gilly]

Originally Posted by FlyingQ

Good stuff. Thanks.

So far I'm happy with the balance of the bike. I don't feel the front or rear wanting to do anything out of the ordinary, although someone who really pushes it would have a more definitive response to how well the balance is in regard to turn in and straight line stability.

[Jonny DeWog]

Originally Posted by direktor

Clearly, the longer the swing arm, the faster and better the bike handles:

Hmmmm......

long swingarm = good.
long wheelbase = bad....well it's a little more complicated than that....

Actually most swingarms are set below the horizontal datum line, such that as they compress - initially - they effectively lengthen the wheelbase. Once they go beyond that (as with long-travel suspensions like dirtbikes etc) they then keep swinging upwards and reduce the wheelbase.

This is purely designed to aid the way a bike is ridden out of corners: As you accelerate hard on the throttle, the rear will squat, and the wheelbase lengthens, needing a wider arc to turn..... not an issue as you exit a corner and the road straightens up. If the opposite were true (swingarm set above true horizontal) then the bike still squats under acceleration, but with a reduced wheelbase, 'tightening' the corner arc instead of relaxing it.....bad news!!


jdw

[couchcommando]

The SF swingarm is the same as the 1098R which was homologated for racing, it was needed in both cases to control wheelies and help with traction, it's obvious if we had a 1098 swingarm and sitting upright like we do that wheelies would be even more of a problem hence the longer arm